A continuous rotation servo is a compact motor module that looks like a hobby servo but spins like a small geared motor. It is useful in robotics because it combines a DC motor, gear train, and control electronics in one easy-to-mount package. Instead of moving to a fixed angle, it turns forward or backward at a speed set by a control pulse.
This makes it common in small wheeled robots, conveyor models, and rotating mechanisms.
The control signal is usually a pulse-width modulation signal with pulses about every 20 ms. A pulse near 1.5 ms commands stop, shorter pulses command rotation one way, and longer pulses command rotation the other way. The servo electronics compare the signal to a neutral point and drive the internal DC motor through gears to produce useful torque at the output shaft. Because it is designed for continuous spin, it does not know its exact angular position unless extra sensors are added.
Understanding Robotics: Continuous Rotation Servo
Inside a continuous rotation servo, the original position feedback system has been changed or disconnected. In an ordinary servo, a small variable resistor reports the output shaft angle to the control circuit. The circuit keeps driving until the measured angle matches the requested angle.
A continuous rotation servo removes that useful angle measurement. Its feedback is set near a middle value instead. The control circuit then treats commands away from the middle as a request to keep the motor moving.
This modification is clever, but it creates an important limit. The robot controller cannot tell how far the wheel has actually turned from the servo signal alone.
The stop command needs careful attention. The stated neutral pulse is only an approximate starting point. Manufacturing differences, temperature, wear, and supply voltage can make one servo creep at the expected neutral setting.
Many models include a tiny adjustment screw for setting the true stop point. Students should calibrate each servo before building a robot. Lift the drive wheels off the table, send the neutral command, and adjust or record the value that gives a reliable stop.
Near neutral, there is often a small dead band where the servo does not move. Outside that region, speed may increase unevenly rather than in perfectly equal steps.
A servo does not produce the same speed in every situation. A wheel rolling freely needs little force, while a robot starting from rest needs more force. Carpet, loose wires, a heavy chassis, and a low battery all increase the load.
As load rises, the motor slows and draws more current. If the load becomes too large, the output shaft can stall. During a stall, the motor may heat up quickly and the battery voltage may drop.
That voltage drop can reset a microcontroller or make sensors give unreliable readings. A good build uses a battery supply that can provide enough current, solid wiring, and a shared electrical ground between the servo supply and the control board.
Wheel motion introduces another source of error. In theory, robot speed equals wheel circumference times the number of wheel rotations each second. Real wheels can slip, have slightly different diameters, or press against the floor with unequal force.
Two servos given the same command may therefore make a robot curve instead of travel straight. A simple test is to mark a measured path on the floor and tune the left and right commands until the robot follows it closely. For more accurate travel, add wheel encoders.
These sensors count wheel movement and allow the program to correct for slow motors, changing batteries, and uneven surfaces. This is called closed loop control, and it is a major idea in practical robotics.
Key Facts
- A standard servo controls angle, while a continuous rotation servo controls speed and direction.
- Typical control period: T = 20 ms, or frequency f = 1/T = 50 Hz.
- Neutral command is often about 1.5 ms pulse width, which should make the servo stop.
- Pulse width less than 1.5 ms usually spins one direction, and pulse width greater than 1.5 ms usually spins the opposite direction.
- Duty cycle = pulse width / period, so a 1.5 ms pulse in a 20 ms period has duty cycle = 0.075 = 7.5%.
- Wheel speed relates to robot speed by v = 2πrN, where r is wheel radius and N is wheel rotations per second.
Vocabulary
- Continuous rotation servo
- A modified servo motor that rotates continuously, with its speed and direction controlled by pulse width.
- Pulse-width modulation
- A control method that changes the width of repeated electrical pulses to send a command to a device.
- Neutral pulse
- The pulse width that tells a continuous rotation servo to stop spinning, usually near 1.5 ms.
- Gear train
- A set of gears that reduces motor speed while increasing torque at the output shaft.
- Torque
- A twisting force that causes rotation, measured in units such as newton-meters or kilogram-centimeters.
Common Mistakes to Avoid
- Treating a continuous rotation servo like a position servo is wrong because pulse width controls speed and direction, not a target angle.
- Assuming 1.5 ms always stops the servo exactly is wrong because the neutral point can vary and may need adjustment or calibration.
- Powering the servo from a microcontroller signal pin is wrong because the signal pin cannot supply the motor current needed by the servo.
- Forgetting a common ground is wrong because the servo and controller need the same voltage reference for the signal pulses to be interpreted correctly.
Practice Questions
- 1 A controller sends pulses every 20 ms. What is the frequency of the PWM signal in hertz?
- 2 A continuous rotation servo receives a 1.2 ms pulse every 20 ms. What is the duty cycle as a percent, and is the command likely stop, forward, or reverse if 1.5 ms is neutral?
- 3 A two-wheel robot uses continuous rotation servos. Explain why matching the same pulse width on both servos may not make the robot drive perfectly straight.